In recent years, in accordance with the advance in optical technology, various types of display technologies such as plasma display panels (PDP), liquid crystal displays (LCD), organic/inorganic electroluminescent displays (ELD) and the like have been suggested and sold in the market instead of a known cathode-ray tube. In the above-mentioned displays, the use of various types of plastic films has been suggested and the required characteristics thereof have been sophisticated. For example, in the case of liquid crystal displays, in order to obtain product slimness and lightness as well as to improve display characteristics, various types of plastic films are used in a polarizing plate, a retardation film, a plastic substrate, a light guide plate, and the like.
In general, the polarizing plate has a structure in which the triacetyl cellulose film (hereinafter, referred to as TAC film) as a protective film is laminated by using an aqueous adhesive made of a polyvinyl alcohol aqueous solution on a polarizer. However, both the polyvinyl alcohol film used as the polarizer and the TAC film used as the protective film for a polarizer have poor heat and humidity resistance. Therefore, if a polarizing plate which is made of the films is used in a high temperature or high humidity atmosphere for a long period of time, the degree of polarization may be reduced, the polarizer and the protective film may be separated from each other, or optical properties may be reduced. Thus, in terms of the purposes, there are various limits. In addition, in the case of the TAC film, a change in in-plane retardation (Rin) and thickness retardation (Rth) is significant according to a change in the ambient temperature/humidity atmosphere. In particular, a change in retardation in respect to incident light at an inclined direction is significant. If a polarizing plate that includes a TAC film having the above-mentioned characteristics as a protective film is applied to a liquid crystal display, the viewing angle characteristics are changed according to the change in the ambient temperature/humidity of the surrounding atmosphere, thus reducing image quality. In addition, in the case of the TAC film, a change in dimension is significant according to the change in the ambient temperature/humidity of the surrounding atmosphere and the photoelastic coefficient is relatively high, thus, after evaluation in respect to durability in a heat resistant and humidity resistant atmosphere, a change in retardation characteristics occurs locally, thereby easily reducing the image quality.
As a material used to avoid the disadvantages of the TAC film, a (meth)acrylic resin is well known. However, it is known that (meth)acrylic resin is easily broken or split, causing problems in terms of transportability during the production of the polarizer plate and that the productivity thereof is poor. In addition, when an acrylate resin is used as a material of a film, it is necessary that a casting process be used, and thus there are problems in that it is difficult to perform the process and the costs are high.
In the case of the retardation film, the film that is made of a styrene resin is a material that shows optical anisotropic properties that the refractive index is increased in a direction perpendicular to the alignment direction when the film is stretched and aligned, and it is known that the film is stretched to be used for manufacturing the film having the positive thickness retardation (Rth). In addition, there are advantages in that the styrene resin has excellent economic efficiency and transparency. However, there are problems in that the heat resistance is insufficient and the mechanical properties are poor except for when costly special monomers are used together to perform the manufacturing process.
In addition, a biaxially-stretched polycarbonate film or a film including a uniaxially-stretched cyclic olefin polymer (COP) on which liquid crystal is coated is used as a compensation film or a retardation film. However, in the case of the biaxially-stretched polycarbonate film, there are problems in that the retardation varies depending on a wavelength of incident light, leading to imperfect retardation compensation. In the case of the liquid crystal coated COP, there are problems in that the price is high and the processes are complex.
Although an acrylic copolymer that has excellent heat resistance and transparency has been studied in order to solve the above-mentioned problems, sufficient impact resistance cannot be attained by using only the acrylic copolymer, and the copolymer may be easily split or broken. Thus, various types of tenacity modifiers are added to secure impact resistance. In this case, if a large amount of a tenacity modifier is used for sufficient impact resistance, heat resistance and transparency are reduced. If the molecular weight of the tenacity modifier is increased, it is difficult to produce and process the copolymer.